WASHINGTON (Reuters) - U.S. President Barack Obama urged Russian President Vladimir Putin on Monday to support peace in Ukraine and said Moscow would face further "costs" if it did not take steps to reduce tensions in the country, the White House said on Monday.

"The president spoke to President Putin and once again urged him to support peace instead of allowing the provision of arms and materiel across the border and continuing support for militants and separatists who are further destabilizing the situation in Ukraine," White House spokesman Josh Earnest told reporters.